In the past, standard backer rods for sealant control have been developed from closed-cell polyethylene foam. These standard backer rods are extruded round rods of low-density, polyethylene (LDPE) material with a self-skinned outer covering. Prior to a sealant being applied, backer rods are inserted into joints between two cured masonry sidewalls. The backer rod may be compressively inserted into the joint and is installed at a depth, typically, of about one-half the width thereof.
After the backer rod is inserted in the expansion joint, the cavity remaining above the backer rod is filled with a sealant material. In practice, the cavity is slightly overfilled with the sealant and the excess is removed in an additional installation step. With the impact of usual ambient conditions over time, the closed cells and their outer skin may become ruptured, which results in trapped gasses escaping from the backer rod foam to the surface of the sealant. This phenomenon is often referred to in the art as “outgassing” and frequently disturbs the sealant.
Because the backer rod is generally impervious, the gasses are prevented from going any place other than through the overlying sealant material. Thus, outgassing may cause seepage through the sealant material leading to the formation of voids in the seal. Such voids compromise the integrity of the seal causing leakage, surface problems, and collapsed foam cells. The collapsed cells change the compression and recovery characteristics of the backer rod, which needs to be maintained during the expansion and contraction of the construction elements.
Bubbling at the surface of the sealant occurs when the seal of the backer rod materials in the expansion joint fails, especially as temperatures warm the joint. This may cause the structure being sealed to expand and compress the backer rod, squeezing the entrained air in a tear or puncturing the sealant.
To meet architectural standards and building code requirements, an installed standard backer rod needs to exhibit several specific physical properties, such as inertness, temperature resistance, low water absorption, and insulative qualities.
U.S. Pat. No. 5,277,515 to Hovis (“Hovis '515”) discloses ethylenic polymer foam for a sealed building joint. The material is an extruded, low-density, non-outgassing ethylenic polymer foam containing 30% to 85% open cells and the remainder closed cells with a self-skinned exterior surface. The compression recovery and lower compression resistance of the foam is presented as being superior to similarly comprised ethylenic closed-cell foams. Additionally, the foam is presented as having superior moisture resistance and handling characteristics when compared to open-cell foams. Foams having greater than 50% open cells are generally extruded at temperatures of no less than 190° F. and no more than 420° F.
Accordingly, a material that provides qualities, such as inertness and compression- and temperature-resistance in a backer rod while substantially reducing problems such as outgassing is desired.